Abstract

An investigation of the self-excitation of low frequency (100-600 Hz), longitudinal acoustic modes of a rearward-facing step combustor has been performed. As in combustion instabilities of air breathing propulsion systems, the pressure oscillations are excited by a fluctuating heat release from a flame that is stabilized in a recirculation zone. Flow visualization results and flame radiation intensity data reveal that large vortex structures are responsible for this fluctuating heat release. The vortices are shed at frequencies corresponding to longitudinal acoustic modes of the system or to the first subharmonic of one of the modes.

A series of parametric studies were performed to determine the dependence of the vortex shedding frequency upon the step height, mean flow speed, fuel type, and equivalence ratio. It was discovered that the vortex shedding frequency can shift between modes as a result of changes in the chemical reaction time of the reactants or as a result of changes in the mixing process of the cold reactants with the hot products.

Detailed investigations into the mechanism of sustenance of the oscillations during instability were performed for several operating conditions. The distribution of the combustion associated with vortex shedding was investigated by measuring the radiation intensity from the flame region. These results were used in Rayleigh's Criterion to determine regions of driving and damping of the oscillations.

A one-dimensional linearized acoustic model is used to predict the natural modes of the system and a fluctuating volumetric source is used to model the oscillatory heat release. First, independent driving is applied to determine the system response to driving at different frequencies. One important result is that, the phase difference between the pressure oscillations and the velocity oscillations at the flameholder is very sensitive to the frequency of driving near natural modes of the system.

Finally, a velocity-sensitive volumetric source with a time delay is included as feedback to determine the linear stability characteristics of the system. Various mechanisms leading to nonlinear limit cycle behavior are then discussed and compared to experimental data obtained during transition to instability.